Since fiber reinforced composite materials have high strength and light weight, they are widely used as structural materials for aircrafts, automobiles, or the like, sporting goods such as golf shafts and tennis rackets, and further molded materials for general industrial applications.
Cellulose nanofibers are conventionally used together with carbon fibers, glass fibers, or the like as reinforcing materials for the fiber reinforced composite materials.
Although carbon fibers or glass fibers have very excellent properties as reinforcing fibers, it is pointed out that they require high energy for production or high environmental load due to difficulty of reuse since they use minerals or fossil fuels as materials.
On the other hand, cellulose nanofibers are advantageous in that, not only they exert excellent reinforcing effects as compared with other reinforcing fibers, but also environmental load during production is low since they are produced from cellulose derived from plants, bacteria, or the like, and further combustion residues do not remain at the time of disposal, unlike glass fibers. Therefore, attention has been given to fiber reinforced composite materials using cellulose nanofibers as reinforcing fibers in recent years.
Cellulose nanofibers can be produced by several methods such as mechanical shearing using a homogenizer or the like (Patent Literature 1) and treating a solution containing an ionic liquid (Patent Literature 2).
An aqueous dispersion obtained by dispersing the cellulose nanofibers produced by the methods described above in water has very high viscosity even at low concentration and has poor dispersibility in a water-soluble polymer. Accordingly, mixing cellulose into a polymer at high concentration, which is so-called high concentration filling, has been not easy. Therefore, a method for improving the dispersibility of cellulose nanofibers in a water-soluble polymer by chemical modification of hydroxyl groups on the surface of the cellulose nanofibers with modifying groups or addition of an additive has been employed (Patent Literature 2).
Further, a method in which desired strength of fiber reinforced composite materials is achieved by mixing a low-concentration dispersion of the cellulose nanofibers with a low-concentration polymer solution and thereafter concentrating the mixture is disclosed (Patent Literature 3).